Pluto's Atmosphere is Billowing Away into Space

Pluto just can’t stop blowing our minds. Not only is it a geologically active world with ice mountains the size of the Rockies and frozen plains of methane and nitrogen, we’ve just learned that Pluto’s atmosphere is pouring away into space. It’s leaving a massive tail of charged plasma in its wake.

That’s one of the latest fascinating Pluto dispatches that NASA’s New Horizons science team shared at a press conference this afternoon. Beginning an hour and a half after its closest approach to Pluto on Tuesday, the New Horizon’s Solar Wind Around Pluto (SWAP) instrument began detecting a ‘plasma tail,’ rich in ionised nitrogen molecules that are presumably being stripped away from the dwarf planet’s atmosphere.

The tail, which extends tens of thousands of miles beyond Pluto, is probably the result of solar winds charging up molecules in the planet’s atmosphere and punting them off into space.

Or, in NASA’s words: “What we think is happening is that the solar wind from the sun—charged particles streaming out at supersonic speeds—are interacting with this escaping atmosphere, producing a shock upstream.”

How fast of a leak are we talking? According to Bagenal, Pluto could be losing atmosphere at a rate of 500 tonnes per hour. For comparison, Mars’ thin atmosphere is only escaping into space at a speed of roughly 1 tonne per hour. The substantially faster loss of Pluto’s atmosphere is probably due to the tiny planet’s weak gravity.

But these numbers are just rough estimates at this point. So far, all we know is that there’s a massive, ionised bubble of particles surrounding Pluto, and we can make some good inferences about how it got there. When we get more data back in August, Pluto’s science team will be able to put some hard numbers to this fascinating phenomenon.

So far, Pluto’s atmosphere appears to be pretty symmetric all the way around the planet, extending from the ground to at least a thousand miles above the surface. At its highest altitudes, the atmosphere is dominated by molecular nitrogen. Lower down, we’re starting to catch whiffs of methane, and very near the surface, larger hydrocarbon molecules.

Although there’s still much to learn, the science team is already pondering the implications of the atmosphere’s fast escape. If you add up the current estimated escape rate of Pluto’s atmosphere over the age of the solar system, Bengal says it’s equivalent to thousands of feet of nitrogen ice steaming off the surface. It’s possible that geysers or cryovolcanoes are helping replenish the atmosphere quickly, although we don’t have observational evidence for those phenomena just yet.

If one thing’s clear at this point, it’s that Pluto is way more dynamic than we could have possibly imagined. And that it still has a lot of secrets to give up.

Top image caption: Artist’s concept of the interaction of the solar wind (the supersonic outflow of electrically charged particles from the Sun) with Pluto’s predominantly nitrogen atmosphere. Some of the molecules that form the atmosphere have enough energy to overcome Pluto’s weak gravity and escape into space, where they are ionized by solar ultraviolet radiation. As the solar wind encounters the obstacle formed by the ions, it is slowed and diverted (depicted in the red region), possibly forming a shock wave upstream of Pluto. The ions are “picked up” by the solar wind and carried in its flow past the dwarf planet to form an ion or plasma tail (blue region).